DESCRIPTION (adapted from the Abstract): A single traumatic experience can result in the development of exaggerated and overgeneralized fear, as observed in Posttraumatic Stress Disorder (PTSD). The goals of the proposed research are to identify the brain structures and neurotransmitters that are involved in the reduction of fear and the disruption of fear reduction in conditions like PTSD. Conditioned fear will be measured in these studies using the fear-potentiated startle response in which the amplitude of an acoustically elicited startle reflect is augmented in the presence of a stimuli previously paired with shock. The reduction of fear-potentiated startle will be assessed with a Pavlovian conditioned inhibition procedure. Two experiments will be conducted to identify the locus of action of the conditioned inhibitor. The first will involve lesions of structures known part of the circuitry mediating fear-potentiated startle and measuring their effects on previously acquired conditioned inhibition. The second experiment will examine the effects of conditioned inhibition on chemically activated fear-potentiation. Together, these experiments will shed substantial light on the neural circuitry mediating fear reduction, providing a basis for developing strategies to treat disorders characterized by exaggerated fear responses.